The invention relates to an apparatus for scanning an information carrier, comprising:
displacement means for moving a scanning spot along a path transverse to the tracks of the information carrier,
position information generating means for generating a position signal which is indicative of the position of the scanning spot on the path with respect to a track,
velocity information generating means for generating a velocity signal which is a measure of the velocity of the scanning spot along the path,
a phase-locked loop for generating a corrected position signal which is locked to the position signal,
means for estimating a displacement of the scanning spot on the basis of the corrected position signal.
The invention further relates to a method of operating such an apparatus.
Such an apparatus and such a method are known from DE 40 29 975 A1. The known apparatus is intended for scanning a disc-shaped information carrier so as to inscribe or to read the information carrier. A transducer generates a positional error signal which is a measure of the position of the scanning spot with respect to a track. From the positional error signal a pulsed track crossing signal is derived. A phase-locked loop corrects the track crossing signal so as to generate an error-corrected track crossing signal. This error-corrected track crossing signal is used for calculating the displacement of the scanning spot. In one embodiment the apparatus further has velocity information generating means in order to achieve that the scanning spot is moved in accordance with a predetermined velocity profile.
In practice, the tracks of a data carrier are not always concentric with the axis. In the case of a comparatively slow movement of the scanning spot this causes the scanning spot to be moved alternately in an inward and an outward direction with respect to the tracks. A consequence of this is that tracks are counted more than once, as a result of which the displacement detected for the scanning spot is too large.
It is an object of the invention to provide an apparatus of the type defined in the opening paragraph in which the displacement of the data carrier is detected accurately even in the case of an eccentric data carrier. To this end, the apparatus in accordance with the invention is characterized in that the position information generating means generate a further position signal which is phase-shifted with respect to the position signal, said apparatus assuming a first mode of operation at a comparatively low velocity of the scanning spot, in which first node of operation the displacement of the scanning spot is derived from both position signals and a second mode of operation at a comparatively high velocity of the scanning spot, in which second mode of operation the displacement is derived from the position signal.
During a jump from one track to another track the scanning spot has a velocity which is initially low, increases to a comparatively high velocity, may remain temporarily constant and subsequently decreases again. In the apparatus in accordance with the invention it is achieved that the displacement of the scanning spot is determined accurately during the entire jump. Since the displacement of the scanning spot is derived from the two mutually phase-shifted position signals at a comparatively low velocity, it is possible to detect crossing of a track as well as the direction of this crossing. Thus, the displacement of the scanning spot can be determined comparatively accurately. At comparatively high velocities one of the position signals usually becomes unreliable. However, at comparatively high velocities a possible eccentricity of the tracks no longer affects the direction. The displacement of the scanning spot is then derived accurately from the corrected position signal.
The velocity-determining means can be realized in various manners. Said means can, for example, estimate the velocity of the scanning spot on the basis of the measured displacement. Alternatively, the velocity of the scanning spot can, for example, be estimated from the signals generated for controlling the displacement.
The measures in accordance with the invention are particularly suitable for an apparatus for scanning a disc-shaped information carrier. However, the measures in accordance with the invention can also be applied to other, for example card-shaped, information carriers.
A further object of the invention is to provide a method of operating an apparatus for scanning an information carrier, in which the displacement can be also be determined comparatively accurately in the case of eccentric data carriers. In the method according to the invention a scanning spot is moved along a path from one track to another track of the information carrier, the scanning spot undergoing an accelerated movement in a first stage of the path and deceleration in a second stage of the path, during which movement along said path in succession a displacement of the scanning spot is derived from a first and a second position signal and, after an estimated value of the velocity of the scanning spot has exceeded a predetermined value, is derived from the first position signal and is independent of the second position signal, after which the displacement of the scanning spot is again derived from the first and the second position signal when the estimated value of the velocity has decreased to a value smaller than a predetermined value.